Respiratory Physiology - Diffusion Flashcards
Drive for diffusion of O2 and CO2 across blood gas membrane
Passive diffusion from high partial pressure to low partial pressure
Fick’s law of diffusion through a tissue sheet
Vgas = volume of gas per unit time
A = area
T = thickness
D = diffusivity (diffusion constant)
(P1 - P2) = difference in partial pressure
What provides the strength of the blood-gas membrane
Type 4 collagen band within the extracellular matrix
Otherwise blood-gas barrier would be extremely fragile given how thin it is
Factors which contribute to diffusivity
Sol = Solubility of the gas
root MW = Square root of molecular weight
Molecular weight is included as per the kinetic theory of gases
Kinetic theory of gases
Velocity of gas is inversely proportional to mass
Time blood spends in pulmonary capillary
~0.75 seconds
CO ~ 5-6 L/min, which is ~100 ml/second
Pulmonary capillary volume ~75 ml
Therefore time = volume / flow rate
= 75/100
= 0.75 s
Diffusion limitation (vs Perfusion limitation)
Diffusion properties of gas limit the uptake of the gas
Leads to large difference between alveolar and end capillary partial pressure
Eg carbon monoxide does not diffuse across blood gas membrane and therefore minimal increase in capillary partial pressure of CO occurs
Perfusion limitation (vs Diffusion limitation)
Gas diffuses across blood-gas membrane and therefore large increase in capillary partial pressure of gas. Therefore no further gas uptake occurs due to lack of partial pressure gradient.
Therefore uptake of gas is limited by pulmonary blood flow (perfusion) as higher blood flow would allow for overall higher gas uptake
Leads to very small difference between alveolar and end capillary partial pressure
Eg. Nitrous oxide
In normal circumstances, what limits O2 uptake across blood-gas membrane
Perfusion limited
Venous PO2 is not at zero, but PO2 rises to almost match alveolar partial pressure
Effect of exercise on diffusion limitations
Exercise increases pulmonary blood flow, therefore reducing time available for gas uptake at the blood-gas membrane
Therefore exercise exacerbates hypoxaemia
Effect of reducing alveolar PO2 on diffusion limited gas
Lower alveolar PO2 results in lower partial pressure gradient, therefore increasing time required to equilibrate partial pressures
Diffusing capacity
Combination of D x A/T as in practice cannot measure area or thickness
Denoted as DL
Measuring diffusing capacity of carbon monoxide
Use of diffusing capacity of carbon monoxide
Helps identify thickened blood-gas membranes (eg pulmonary fibrosis) as CO is diffusion limited so if worsening it indicated worsening diffusion capacity of blood-gas barrier
Method of measuring diffusing capacity of carbon monoxide
Why is carbon monoxide used for diffusion capacity
It is significantly diffusion limited
Resistances to uptake of O2
Diffusion across membrane and contents of pulmonary capillary
Time delay for oxygen to react with haemoglobin
How to measure time for oxygen to react with reduced Hb
Motor driven syringes with reduced Hb and O2 solution
Oxy-haemoglobin measured with spectrophotometry
Percentage Oxyhaemoglobin plotted against time
Components of diffusing capacity
DM = diffusing capacity of membrane (physical process of diffusion)
Theta = rate of reaction of O2 with Hb
Vc = volume of blood in pulmonary capillary
Units of Theta
ml of O2 / min / mmHg / ml of blood
Units of diffusing capacity
ml / min / mmHg
Why is the diffusing capacity equation using inverses
DL is a conductance and you cannot add conductance
Resistance is the reciprocal of conductance
Resistances can be added
Is transfer of CO2 ever limited by diffusion?
Not entirely known but general consensus is that it is not limited by diffusion
